Nickel Complexes of Bidentate [S₂] and [SN] Borato Ligands

Nickel(II) complexes of the monoanionic borato ligands [Ph2B(CH2SCH3)2] (abbreviated Ph2Bt), [Ph2B(CH2StBu)2] (Ph2BttBu), [Ph2B(1-pyrazolyl)(CH2SCH3)], and [Ph2B(1-pyrazolyl)(CH2StBu)] have been prepared and characterized. While [Ph2Bt] formed the square planar homoleptic complex, [Ph2Bt]2Ni, the larger [S2] ligand with tert-butyl substituents, [Ph2BttBu], yielded an unexpected organometallic derivative, [Ph2BttBu]Ni(η2-CH2SBut), resulting from B−C bond rupture. The analogous thiametallacycle derived from the [S3] ligand, [PhB(CH2StBu)3] (PhTttBu), has been structurally authenticated (Schebler, P. J.; Mandimutsira, B. S.; Riordan, C. G.; Liable-Sands, L.; Incarvito, C. D.; Rheingold, A. L. J. Am. Chem. Soc. 2001, 123, 331). The [SN] borato ligands formed exclusively the cis stereoisomers upon reaction with Ni(II) sources, [Ph2B(1-pyrazolyl)(CH2SR)]2Ni. Analysis of the Ni(II/I) reduction potentials by cyclic voltammetry revealed a ∼600 mV anodic shift upon replacement of two thioether donors ([Ph2Bt]2Ni) with two pyrazolyl donors ([Ph2B(1-pyrazolyl)(CH2SCH3)]2Ni) consistent with the all thioether environment stabilizing the lower oxidation state of nickel.